Thumbs up for this topic! To calculate Total Space Used on all filesystems, into single aggregate DCI:
			Code Select 
UsedSpaceTotal=0;
foreach (dci : FindAllDCIs($node,"FileSystem.Used*") )
{
	dciValue = GetDCIValue($node,dci->id);
	if ( dciValue == null ) continue; // To skip in case of errors
	UsedSpaceTotal+=dciValue;	
}
return UsedSpaceTotal;